1. Field of the Invention
The present invention relates to an image forming apparatus in which an image is formed by uniformly charging an image carrier, exposing an image in accordance with inputted image data, and changing a potential on the image carrier. The present invention also relates to a correction method for making the charge potential of the image carrier uniform, and for making the exposure potential of the image carrier uniform when the image is exposed.
2. Description of the Related Art
Heretofore, copying machines, laser beam printers and the like, which employ electrophotography, are known as high-speed, high-quality image forming apparatuses. In recent years, with the advancement of digital technology, a shift from monochrome to color prints and the improvement of qualities of output images have been in rapid progress. Above all, in the field of DTP, there is a strong demand for the color stability and on-surface uniformity of output objects, and various calibration technologies and various technologies for realizing stabilization of electrophotographic processes have been disclosed.
Factors which impair the color stability and on-surface uniformity in an output object, i.e., on the surface of an output image, include, for example, the film thickness unevenness and sensitivity unevenness of an image carrier, the longitudinal unevenness of a charger, the longitudinal unevenness and sleeve revolution unevenness of a developing unit, and various other kinds of unevenness in transfer and fusing. Since these factors occur in combination, various correction technologies have been disclosed. Above all, many correction technologies have been disclosed for unevenness caused by an image carrier, because the pattern of this unevenness is intrinsic to a photoconductor and is therefore relatively stable, and it is difficult to reduce film thickness unevenness and sensitivity unevenness for manufacturing reasons.
Here, unevenness caused by an image carrier will be described from the viewpoints of the constitution and the manufacturing method of the image carrier.
As the image carrier, a function separation type or a single-layer type are used. The function separation type has a two-layer structure including a charge generation layer and a charge transport layer on a conductive supporting base as a lowest layer. With regard to a material constituting the photoconductor, an organic photoconductor (hereinafter referred to as an OPC) made of an organic matter or a photoconductor, which is called an inorganic photoconductor, made of selenium (Se) or silicon (Si) can be used.
In a method of manufacturing an OPC, a solution having a raw material for the OPC dissolved therein is sequentially applied to a base. As the method of manufacturing an OPC, it is possible to use a method such as a spray coating method in which the solution is applied by spraying, and a dipping method in which a base immersed in a solution is extracted to form a film. The thickness of the film in this case and the quality thereof such as the raw material density of the film are adjusted by controlling the viscosity of the solution used to form the film and the extraction speed for dipping. In a case where film characteristics at that time are not uniform, unevenness in the potential on the surface of the photoconductor after charging and unevenness in the exposure potential after exposure occur. Furthermore, in a case where there is unevenness in hardness, charge potential unevenness and exposure potential unevenness occur due to wear unevenness caused by repeating outputs.
As a method of manufacturing an inorganic photoconductor, e.g., an amorphous silicon photoconductor, deposition methods such as vacuum evaporation, sputtering, ion plating, thermal CVD, photo CVD and plasma CVD can be used as described in Japanese Patent Application Publication No. 60-035059 (1985). Among these, plasma CVD in which source gas is decomposed by a direct current, high frequency, or microwave glow discharge to form an a-Si deposit on a supporting base has been put into practical use as a suitable one. In a case where a photoconductor film is formed using such a deposition method, unevenness in film thickness and film quality occurs as in the case of OPCs. Thus, charge potential unevenness and exposure potential unevenness occur on the surface of the photoconductor.
Moreover, as described in Japanese Patent Application Laid-open No. 60-067951 (1985), there is also a photoconductor which achieves improvements such as increasing the strength of a film by superposing a translucent insulating overcoat layer thereto to lengthen the life for the case of repeating outputs. In the case where such improvements are made, charge potential unevenness and exposure potential unevenness on the surface of the photoconductor further increase due to unevenness in the thickness and quality of the added film.
As described above, it is inevitable that an image carrier has unevenness on the surface thereof, and various correction technologies have been contrived. For example, Japanese Patent Application Laid-open Nos. 63-049778 (1988) and 63-049779 (1988) disclose a technology for making the potential (exposure potential) of a laser-exposed portion of a photoconductor uniform along the axial direction thereof by correcting the lighting time of a laser depending on potential characteristics of the exposed portion. This can be achieved by correcting a PWM signal by using a table corresponding to exposure potential characteristics.
Japanese Patent Application Laid-open No. 2000-267363 discloses a technology for correcting exposure by performing exposure with a constant light quantity after charging and then measuring sensitivity unevenness along the direction of movement of a photoconductor by using a potential sensor. In this correction method, correction exposure as an 8-bit laser power value for each pixel is converted into an analog voltage by a digital-to-analog converter, and a voltage value obtained by comparing this voltage and a reference voltage is inputted to the base of a transistor, thereby determining a laser driving current value corresponding to the laser power value. Thus, similar effects can be obtained.
In Japanese Patent Application Laid-open Nos. 5-188707 (1993) and No. 2002-067387, a technology is described in which a latent image region on a photoconductor is divided into two-dimensional segments to perform correction for each segment. In Japanese Patent Application Laid-open Nos. 5-165295 (1993), 5-224483 (1993), 6-003911 (1994), 6-011931 (1994), 6-130767 (1994), 6-266194 (1994) and 2004-258482, described are methods of measuring the sensitivity unevenness of a photoconductor by using a movable potential sensor/density sensor, a plurality of potential sensors/density sensors, or the like. Japanese Patent Application Laid-open No. 2004-223716 discloses a laser control method in which sensitivity unevenness on the entire surface of a photoconductor is corrected.
As described above, many technologies have been disclosed with regard to uniformity in the plane of an image, particularly unevenness on an image carrier. However, in most of the technologies, a single unevenness is corrected. Moreover, even in technologies in which a plurality of kinds of unevenness are corrected, the plurality of kinds of unevenness are corrected together without separating factors of the unevenness, and the current situation is that sufficient correction cannot be realized.
For example, as shown in FIG. 1, there is a case where a charge potential 101 on the surface is flat and does not need to be corrected, and where an exposure potential 102 is uneven and needs to be corrected. In such a case, a flat exposure potential 202 as shown in FIG. 2 can be realized by adjusting the intensity of exposure by multiplying it by a needed correction coefficient for each area on the surface in order that unevenness in the exposure potential can be made flat.
As shown in FIGS. 3 and 4, there is also a case where unevenness in a charge potential 401 and unevenness in an exposure potential 402 occur simultaneously. For example, a characteristic curve (hereinafter referred to as a V-E curve) is referred to in which the integrated exposure (energy) and the surface potential (voltage) at the time are respectively plotted on the horizontal and vertical axes as shown in FIG. 5. For the purpose of making the potentials at A-point and B-point equal to the same potential 501, the desired potential 501 can be obtained at the same pulse width as A-point as shown in the right graph of FIG. 5 by, for example, adjusting the intensity of exposure of a laser. However, charge potential unevenness needs to be corrected separately.
If average correction is targeted, it is also possible to perform correction with the potential unevenness 506 of a halftone region 505 typifying unevenness information. However, in this case, the exposure and charge potentials cannot be corrected with good consistency. In the left graph of FIG. 5, integrated exposure is shown on the horizontal axis in order to show an original V-E curve. In the right graph of FIG. 5, the result of replotting the foregoing by assuming input data is shown.
As shown in FIG. 4, there is a case where unevenness in different characteristics occurs in combination in each area on the surface. In such a case, it is possible to obtain unevenness information for all tones for each area and perform correction for each area. However, this requires not only a huge memory area for storing unevenness information for all tones for each area but also many measurements for obtaining the unevenness information, and therefore leads directly to an increase in cost and a reduction in throughput. Thus, it is a very difficult problem to correct kinds of unevenness, which have different characteristics, simply and with good consistency.